Discussion
Many countries in tropical Asia are struggling to conduct influenza surveillance in 2020, likely due to disruptions from the COVID-19 pandemic. However, unlike the observation in the Southern Hemisphere1,6,7 influenza has not disappeared from circulation, and some countries experienced a delayed resurgence of community circulation of influenza viruses. The heterogeneity in the magnitude of the influenza activity across countries in tropical Asia is noteworthy and likely due to multiple factors, including surveillance artefact, degree of travel restrictions, and adherence to COVID-19 interventions. Although data from Africa are sparse, a similar pattern may be emerging.5 This has important implications for the 2020-2021 Northern Hemisphere influenza season, and for future influenza vaccine strain selection.
First, it is critical that persons recommended to receive influenza vaccinations get vaccinated. The COVID-19 pandemic is disproportionately impacting older persons, those with underlying chronic conditions, and other traditionally underserved populations,8,9 many of whom are also at increased risk of severe complications of influenza. Influenza vaccination could prevent unnecessary added morbidity and mortality in these target groups and may prevent health care facilities from being overwhelmed by persons concerned that influenza-associated respiratory symptoms may represent COVID-19 illness.
Second, maintaining surveillance and outbreak response is essential to track the geographic spread and obtain viruses so vaccines remain optimized to circulating influenza viruses. At the WHO Collaborating Center for Reference and Research on Influenza at CDC Atlanta, global surveillance submitted 583 influenza viruses to CDC between March 1 and September 30, 2020, a 69% decrease from an average of 1,908 viruses during the same months in the years 2015-2019. Furthermore, the limited data from tropical Asia suggest that genetically divergent influenza A(H3N2) viruses are currently in circulation. In Cambodia, officials investigated outbreaks of influenza A(H3N2) viruses in semi-closed settings in August-October 2020.10 These viruses and some from Bangladesh, belong to a different genetic clade, 3C.2a1b+T131K, than the currently recommended 2020-2021 Northern Hemisphere influenza vaccine virus, A/Hong Kong/267/2019, which comes from genetic clade 3C.2a1b+135K+137F.11
The data presented here are a reminder that the low levels of influenza circulation seen in the northern hemisphere in summer 2020 may not necessarily persist into the upcoming influenza season, and influenza surveillance and prevention strategies should continue as planned and not be delayed.